“Nuclear terrorism” has become an alarming possibility and countries are not doing enough to prevent it, the head of the UN atomic watchdog warned yesterday.

In the wake of claims the Brussels attackers had planned to set off a radioactive ‘dirty bomb’, Yukiya Amano, the director general of the International Atomic Energy Agency said: “Terrorism is spreading and the possibility of using nuclear material cannot be excluded.

“Dirty bombs will be enough to (drive) any big city in the world into panic. And the psychological, economic and political implications would be enormous”

Yukiya Amano, the director general of the International Atomic Energy Agency

“Member states need to have sustained interest in strengthening nuclear security. The countries which do not recognise the danger of nuclear terrorism is the biggest problem.”

Mr Amano said it is “not impossible” that extremists could manage to make a “primitive” nuclear bomb - if they got hold of the material. But he said that a far likelier risk was a “dirty bomb”, in which conventional explosives are used to disperse radioactive material.

The material can be found in small quantities in universities, hospitals and other facilities. “Dirty bombs will be enough to (drive) any big city in the world into panic. And the psychological, economic and political implications would be enormous,” said Mr Amano.

The Telegraph understands that Belgian’s nuclear bosses have ordered an urgent review of staff records over fears jihadi terrorists had infiltrated the industry.

One security expert suggested that the terrorists could have been plotting to kidnap the nuclear researcher they had been filming with a view to coercing the scientist into helping them make a ‘dirty bomb’.

The researcher is reported to have worked at a facility which stores a “significant portion of the world’s supply of radioisotopes”.

Claude Moniquet, who runs the European Strategic Intelligence and Security Center, said: “We can imagine that the terrorists might want to kidnap someone or kidnap his family. The terrorist cell ... naively believed they could use him to penetrate a lab to obtain nuclear material to make a dirty bomb.”

Eleven people who worked in Belgium’s Tihange nuclear power plant have been barred access to the sensitive site due to heightened security over the terror threat.

Mourners gather on Place de la Bourse, Brussels, after the attacks

It is not the first time the concerns over radicalised staff has surfaced. In 2013, an engineer from Doel 4, a nuclear reactor near Antwerp, was fired. His brother-in-law was a member of a group of notorious Islamist extremist group called Sharia4Belgium, who had joined Islamic State in Iraq and the Levant.

Terrorists were also suspected of sabotaging Doel 4 in 2014 that led to it being shut down for more than four months. The federal prosecutor at the time refused to rule out terrorism.

It has also been claimed the Brussels suicide bombers had been planning an attack on a nuclear power plant but switched to an airport and train station at the last minute.

The target was changed after one of the terror cell’s ringleaders was captured in a police shootout a week before Tuesday’s bombings.

"we can say that their scale could have been much greater if the terrorists had managed to carry out their initial plan and didn’t opt for easier targets."

Police source

It is thought the terrorists switched to the ‘softer’ targets amid fears the net was closing in.

It was alleged that two members of the cell - Ibrahim and Khalid el-Bakraoui, who detonated two of the bombs - had been involved in the covert filming of the daily routine of an unidentified senior official in Belgium’s nuclear research and development programme.

More than 10 hours of footage was recovered in December from an apartment in Brussels that was raided by police in the aftermath of the attacks on Paris a month earlier. The apartment in Schaerbeek neighbourhood had been used as a bomb making factory.

It was thought the footage was shot by Mohammed Bakkali, who was arrested in November for his crucial role in the Paris attacks, but had passed the footage on to the brothers.

DH, the Belgian magazine, reported that the brothers had picked up the covert footage shortly after the Paris attacks and that the film contained “ten hours of rushes”.

A police source told the magazine: “We knew that this [attack] was going to happen. They no doubt brought forward their operations because they felt under pressure. Even if we didn’t managed to avert these attacks, we can say that their scale could have been much greater if the terrorists had managed to carry out their initial plan and didn’t opt for easier targets.”

Seven nuclear plant workers had their security clearance removed following a raid on another part of the terror cell on March 15 at a house in the Forest district of Brussels. In that raid police killed one suspect, Mohamed Belkaid, leading to the arrest of Salah Abdeslam, another key member of the cell, three days later. Abdeslam was in custody and said to be co-operating with the authorities, prompting the brothers to bring forward their deadly plot and switch targets.

According to RTBF, the state broadcaster, a further four nuclear plant workers were banned entry after Tuesday’s devastating bomb blasts in an airport and metro station.

In the wake of Abdeslam’s arrest, a detachment of soldiers was sent to guard Tihange - just after investigators made the connection between the safe house and the El Bakraoui brothers.

Investigators reportedly are looking into whether there was a dual plot between Bakkali and the El Bakraoui brothers.